Various hydrocarbon gases are generated in the arts of petroleum refining or petrochemistry. However, these gases are not necessarily utilized efficiently as source material gases for various substances, so that a method of conversion into more effective substances has been demanded.
For manufacturing a synthesized gas containing hydrogen and carbon monoxide by reforming a hydrocarbon gas, there are known methods such as reforming a hydrocarbon with carbon dioxide, reforming a hydrocarbon with water vapor, and reforming saturated hydrocarbon by using the carbon dioxide and water vapor in combination in which both of carbon dioxide and water vapor are allowed to react in the presence of a catalyst.
Reforming a hydrocarbon with carbon dioxide is suitable for manufacturing a synthesized gas having a comparatively high carbon monoxide concentration by allowing a saturated hydrocarbon such as methane and carbon dioxide to react in the presence of a catalyst.
On the other hand, reforming a hydrocarbon with water vapor is suitable for manufacturing a synthesized gas having a comparatively high hydrogen concentration by allowing a saturated hydrocarbon such as methane and water vapor to react in the presence of a catalyst.
Also, the method of reforming a saturated hydrocarbon such as methane by using carbon dioxide and water vapor in combination in which both of carbon dioxide and water vapor are allowed to react in the presence of a catalyst has an advantage in that the ratio of hydrogen and carbon monoxide in the synthesized gas to be manufactured can be adjusted by adjusting the ratio of carbon dioxide and water vapor.
By such reforming of a hydrocarbon gas, carbon may be deposited on the catalyst during the process of decomposition of the hydrocarbon. The degree of this carbon deposition varies depending on a hydrocarbon reforming conditions. It is reported that carbon is most liable to be deposited in the reforming of a hydrocarbon with carbon dioxide, and that the amount of carbon deposition is comparatively small in the reforming of a hydrocarbon with water vapor. However, the carbon deposited on the catalyst gradually accumulates to lower the catalyst activity. When carbon is deposited in a large amount, there is a fear of clogging the reaction tube. Therefore, even in the reforming of a hydrocarbon with water vapor, carbon deposition is restrained generally by setting the ratio of water vapor to hydrocarbon (hereafter “water vapor/hydrocarbon ratio”) to be high in order to introduce water vapor in an excessive amount.
As a catalyst for reforming a hydrocarbon with carbon dioxide or water vapor, there are known a nickel catalyst in which nickel is carried on a base such as alumina, and a supported ruthenium catalyst (See Patent Document 1), and further a rhodium catalyst in which rhodium is carried on a base such as alumina (See Patent Document 2), and the like.
Also, as a catalyst for reforming hydrocarbon with carbon dioxide, a catalyst containing a carbonate of at least one kind of alkaline earth metal selected from Ca, Sr, and Ba, a catalyst metal selected from Ni, Rh, Ru, Ir, Pd, Pt, Re, Co, Fe, Mo, and the like, and ATiO3 (A is at least one kind of alkaline earth metal selected from the group consisting of Ca, Sr, and Ba) is proposed (See Patent Document 3).
A typical nickel catalyst for reforming a hydrocarbon with water vapor in which nickel is carried on a base such as alumina is liable to cause carbon deposition on the catalyst. Therefore, there is the need to perform a reaction of reforming hydrocarbon with water vapor under a condition of a high water vapor/hydrocarbon ratio in which water vapor is excessive relative to the hydrocarbon in order to restrain the lowering of activity by carbon deposition. In order to make the water vapor excessive, however, there is a problem in that the energy consumption increases in the process of vaporizing water. Also, there is a problem in that it is not suitable for a use which needs a synthesized gas having a high carbon monoxide concentration, such as fuel synthesis, because the carbon monoxide concentration in the composition of the synthesized gas to be manufactured decreases. Further, there is a problem in that a stable and efficient operation of the apparatus is difficult with the above nickel catalyst in the case of attempting to reform a hydrocarbon with carbon dioxide or to reform a hydrocarbon by using carbon dioxide and water vapor in combination because it is a reforming reaction that is more liable to generate the carbon deposition.
A ruthenium catalyst as shown in Patent Document 1 has a function of restraining carbon deposition, so that the carbon deposition is less in amount compared to a nickel catalyst, and also maintenance of the activity is easy. However, there is a problem in that when an unsaturated hydrocarbon such as ethylene coexists in a source material, thermal carbon deposition and a decrease in the activity are liable to occur, so that, even if the ruthenium catalyst produces an effect of restraining the carbon deposition, the catalyst is poisoned by unsaturated hydrocarbon or the like contained in the source material gas, leading to a decrease in the activity.
It is assumed that a rhodium catalyst in which rhodium is carried on a base such as alumina, as shown in Patent Document 2, also raises a similar problem.
Even in a case where the catalyst for reforming with carbon dioxide of Patent Document 3 is used, carbon deposition onto the catalyst is liable to be generated when reforming under a high-pressure condition, thereby raising a problem of decrease in the reforming efficiency.